Particle Image Velocimetry (PIV), which acquires a quantitative velocity field by processing a flow image containing displacement information of particles inside a flow, is a measurement technique that is capable of not only providing qualitative instantaneous flow information but also extracting quantitative flow information having high spatial resolution. As for such PIV, PIV in a narrow sense and Particle Tracking Velocimetry (PTV) are mainly used. In PIV in a narrow sense, an instantaneous velocity field is obtained by computing a Fourier transform or a direct correlation coefficient with respect to an intensity profile of dispersion particle images within an interrogating window of flow images. In general, PIV in a narrow sense is applied to a case where particle density is high, and computes quantitative velocity information by extracting a representative velocity of the interrogating window. Unlike PIV in a narrow sense, PTV obtains displacement information of each particle by extracting locations of the particle from a plurality of flow images successively obtained and tracking the particle. FIG. 1 is a diagram showing a basic principle of a PIV velocity field measurement technique using digital image processing. The PIV velocity field measurement technique obtains an instantaneous velocity field by dividing displacements of particles, which have been captured from two particle images obtained at a time interval Δt, by the time interval Δt. The PIV velocity field measurement technique, which will be mentioned in this application, includes both PIV in a narrow sense and PTV.
In the meantime, there are several techniques that are capable of observing interiors of opaque objects that cannot be observed by a naked eye, such as a human body. The techniques include many using X-rays, such as a clinical X-ray and an X-ray for non-destructive inspection. X-ray equipment is being used to visualize internal structures of objects, in various fields, such as a detection of internal structures of various substances, a visualization of an inside of a human body and a non-destructive inspection of defects inside an industrial equipment.
The PIV velocity field measurement technique and the X-ray imaging techniques are excellent techniques, but have the following disadvantages. The PIV technique can measure a quantitative velocity field information of a given flow, but has a limitation in that an experimental model and a working fluid must be all transparent because an image of particles seeded inside a flow must be acquired using visible light such as laser light. That is, it is impossible to measure a flow inside an opaque object or an opaque fluid flow with a conventional PIV technique which employs visible light, such as white light or laser light.
In the meantime, the conventional X-ray imaging techniques can visualize the interiors of objects, which cannot be observed by a naked eye, using transmitting feature of X-rays. However, most testing specimens of the conventional X-ray imaging techniques are solid objects. The conventional X-ray imaging techniques are used for visualizing internal structures of objects and have imperfections in a measurement of fluid flows inside opaque objects. Furthermore, evolution of trace particles tracking a flow causes a difference in phase or density with respect to an X-ray, has hardly progressed. Accordingly, there has been no case where a flow inside an object has been quantitatively measured using the conventional X-ray imaging techniques. Since it has not been long since the PIV velocity field measurement techniques using visible light were developed, and further since the fields, to which the velocity field measurement techniques are applied, and the fields, to which the X-ray imaging techniques are applied, are very different from each other, there has been no attempt to combine the PIV velocity field measurement technique with the X-ray imaging techniques.